The present invention generally relates to a method and apparatus for autopriming a cassette used in a positive displacement volumetric infusion pumping system, and more specifically, to a method and apparatus for minimizing the accumulation of air bubbles in the cassette during the autopriming operation.
Various types of pumps are used by medical personnel to infuse drugs into a patient""s body. Of these, cassette infusion pumps are often preferred because they provide a more accurately controlled rate and volume of drug infusion than other types of infusion pumps. A cassette pump typically employs a disposable plastic cassette coupled in a liquid line extending between a drug reservoir and the patient""s body. The cassette is driven by a pump to infuse liquid from the reservoir through the liquid line.
In one prior art design of a cassette infusion pump, the cassette comprises a plastic shell or housing having a front section joined to a back section. A thin elastomeric sheet or membrane is encapsulated between the two sections. Liquid flows from the reservoir through an inlet port into a pumping chamber defined between the elastomeric membrane and a concave depression formed in the housing. The cassette is inserted into an appropriate receptacle in a pump chassis that typically includes a microprocessor controller and a motor or solenoid-actuated driver. A plunger actuated by the motor or solenoid in the pump driver displaces the elastomeric membrane into the pumping chamber to force liquid from the pumping chamber through an outlet port under pressure. The pump chassis thus provides the driving force that pumps liquid through the cassette. The microprocessor control is programmable to deliver a selected volume of liquid to the patient at a selected rate of flow. In addition, the pump chassis normally includes one or more pressure sensors and air bubble sensors for monitoring and controlling the drug infusion process to protect against potential problems that may arise during the drug delivery.
Many prior art pump systems require manual priming procedures whenever the system is initially connected to the reservoir or supply of liquid, and then again, if a new liquid supply is connected, or if an exhausted liquid supply is replaced. The purpose of priming a pumping system is to ensure that it functions properly, and more importantly, to ensure that air bubbles that can be trapped in a liquid line when initially connected to the cassette do not enter a patient""s bloodstream, since air bubbles can have potentially harmful consequences. However, manual priming procedures are time consuming and labor intensive, and often must be performed by a doctor or nurse, which tends to drive up medical costs. Autopriming systems, such as the system described in commonly assigned U.S. Pat. No. 5,496,273, are known in the prior art.
A common method for priming an infusion pump is to ensure that the distal end of the liquid line is disconnected from the patient, and to actuate the pump until no air is observed in the liquid being distally discharged from that end of the liquid line. While this priming technique is effective, a significant amount of liquid must be used to prime the system. In medical environments, the liquid is often a medical solution containing expensive drugs, and sterility concerns prevent the reuse of the liquid discharged during priming. For this reason, a back priming technique is useful to minimize the amount of liquid lost during priming. Back priming involves introducing liquid into the pump from the liquid supply, and then causing the pump to force the liquid to flow in the reverse direction (proximally), which causes air from the pump to be discharged into the liquid supply. This air travels up through the liquid in the liquid supply and rises to a head space within the top of the container. Back priming is only possible when the volume of liquid that can be moved by a single stroke of the pump is greater than the volume of the liquid line leading from the proximal inlet port of the pump to the liquid supply. By employing back priming, it is possible to minimize waste of the liquid caused by discharging liquid from the distal liquid line that is disconnected from the patient.
While prior art methods of autopriming have been able to remove the majority of air contained within a pump and its associated liquid lines, the autopriming process can generate micro bubbles of air in the liquid during the priming process. This problem arises because during the priming cycles, the agitation of the liquid/air mixture within the pump and the changes in pressure conditions within the pump incident to the pumping process cause small air bubbles to be formed. The surface tension along the internal walls of the cassette can capture a significant volume of these small air bubbles, and removing these bubbles in the priming process is difficult. Over a period of time, such bubbles can coalesce to form larger bubbles and are thus undesirable. Bubbles contained in the air trap of a pump cassette pose little problem, as the purpose of the air trap is to retain bubbles entrained in the liquid being delivered by the pump, and any micro air bubbles adhering to the walls of the air trap are likely to rise to the top of the air trap and be prevented from exiting the pump. However, air bubbles formed in, or migrating to, the pumping chamber of a cassette are more problematic. Such bubbles affect the accuracy of the pumping process, as their presence slightly alters the available liquid volume of the pumping chamber. Furthermore, air bubbles in the pumping chamber can become entrained in the liquid being pumped into the patient. While the relative volume of these bubbles is small, thereby presenting little real risk to the patient, clearly, it would be desirable to provide a method for minimizing the accumulation of air bubbles in the pumping chamber of a cassette pump during an autoprime sequence.
Preferably, such a method would prevent air bubbles from migrating from the air trap to the pumping chamber during the autopriming process, but would allow air bubbles from the pumping chamber to escape into and be retained in the air trap. Such a method would preferably be adaptable to existing pump systems without necessitating additional components, but instead, accomplished by requiring only software modifications. Such a method should preferably employ an empirically determined algorithm that uses real-time measurements of the presence of air within the pump cassette to control the autopriming process in response to conditions within the cassette. The prior art does not provide an autopriming algorithm that minimizes the migration of air bubbles into the pumping chamber of a pump cassette.
In accord with the present invention, a method is defined for priming a cassette pump used for infusing a liquid into a patient so as to minimize the migration of air bubbles into a pumping chamber of the cassette. The method includes the steps of forming a layer of liquid in an air trap of the cassette such that the layer of liquid separates the air trap from the pumping chamber, and back priming the cassette until air is removed from the proximal volume of the system. The cassette is then forward primed until air is removed from the pumping chamber and a distal volume of the system.
In one embodiment, air is removed from the proximal volume by introducing liquid into the cassette and pumping the liquid in a proximal direction until no air is detected by a proximal air sensor within the cassette. Air is then removed from the pumping chamber and distal volume by introducing additional liquid into the cassette and pumping the additional liquid in a distal direction until a desired volume of liquid is distally delivered.
In one embodiment, the liquid layer is created by introducing a full stroke of liquid into a disposable cassette during each pump cycle, until a proximal air sensor within the cassette senses liquid. During the introduction liquid caused by that full stroke, air is distally expelled from the disposable cassette. Pumping is stopped at that point in the pump cycle. More than a full stroke of liquid is delivered into the cassette during the next pump cycle. Preferably, the latter step delivers a volume of liquid that approximates the interior volume of the air trap.
In another embodiment, back priming is accomplished by introducing liquid into the cassette, and pumping that liquid in a proximal direction until no air is detected by a proximal air sensor within the disposable cassette, using full pump strokes of liquid during each pump cycle. Preferably, back priming continues until no air is detected by the proximal air sensor, and then one additional full pump stroke of liquid is back primed to ensure that any air within the proximal volume is discharged into a liquid supply that is in fluid communication with the proximal volume. For back priming to be successful, the volume of a full pump stroke of liquid should exceed the proximal volume.
With respect to forward priming and removing air from the pumping chamber and the distal volume, in one embodiment, a full pump stroke of liquid is distally delivered in each pump cycle. In another embodiment, if the distal volume exceeds the volume of a full pump stroke of liquid, after a full pump stroke of liquid is delivered, the next pump stroke is modified to deliver only enough liquid to make up the difference between a full pump stroke of liquid and the distal volume. In yet another embodiment, when the distal volume is less than the volume of a full pump stroke of liquid, forward priming is accomplished by using a full pump stroke of liquid. In still another embodiment, when the distal volume is less than the volume of a full pump stroke of liquid, forward priming is accomplished by using a partial pump stroke of liquid.
Preferably, the pump being primed includes a housing that defines a liquid path between an inlet port adapted to couple in fluid communication with a source, and an outlet port adapted to couple in fluid communication with an infusion site on the patient. Also, the pump preferably includes an inlet air sensor that produces a signal indicative of air being detected proximate to the inlet port, the liquid path including a pumping chamber covered by an elastomeric membrane that when forced into the pumping chamber by a driven member, displaces the liquid from the pumping chamber through one of the inlet port and the outlet port; and an air trap that is preferably disposed between the inlet air sensor and the pumping chamber. By introducing sufficient liquid into the air trap of the cassette, a layer of liquid separates the interior volume of the pumping chamber from the interior volume of the air trap, acting as a one-way valve that allows air from the pumping chamber to pass into the air trap, while preventing air from the air trap from passing into the pumping chamber. Preferably, the layer of liquid substantially covers the bottom of the air trap. In one embodiment, the layer of liquid is established by pumping in a forward direction, using strokes that deliver a standard volume of liquid in each pump cycle, until the inlet air sensor does not detect air; and then pumping one additional cycle in the forward direction, using an extra long stroke that delivers a larger than standard volume of liquid in the additional cycle(s). Preferably, the volume delivered by the extra long stroke(s) substantially equals the interior volume of the air trap. In another embodiment, the volume delivered by the extra long stroke(s) is empirically determined to generate a layer of liquid within the air trap that is of a desired size.
Another aspect of the present invention is directed to apparatus that includes elements that perform functions generally consistent with the steps implemented by the method described above.